Mechanical-activation-triggered gibbsite-to-boehmite transition and activation-derived alumina powders

Mechanical activation of monoclinic gibbsite (AI(OH),) in nitrogen led to t he formation of nanocrystalline orthorhombic boehmite (ALOOH) at room tempe rature. The boehmite phase formed after merely 3 h of mechanical activation and developed steadily as the mechanical-activation time increased. Forty hours of mechanical activation resulted in essentially single-phase boehmit e, together with or-alumina (alpha -Al2O3) nanocrystallites 2-3 nm in size. The sequence of phase transitions in the activation-derived boehmite was a s follows: boehmite to gamma -Al2O3 and then to alpha -Al2O3 when flash-cal cined at a heating rate of 10 degreesC/min in air, gamma -Al2O3 formed at 5 20 degreesC, and flash calcination to 1100 degreesC led to the formation of an alpha -Al2O3 phase, which exhibited a refined particle size in the rang e of 100-200 nm, In contrast, the gibbsite-to-boehmite transition in the un activated gibbsite occurred over the temperature range of 220 degrees -330 degreesC, A flash-calcination temperature of 1400 degreesC was required to complete the conversion to alpha -Al2O3 phase, with both delta -Al2O3 and t heta -Al2O3 as the transitional phases. The resulting alumina powder consis ted of irregularly shaped particles 0.4-0.8 mum in size, together with an e xtensive degree of particle agglomeration.